1. Field of the Invention
The present invention relates to a fluid pump from which fluid is discharged by utilizing the deformation of a piezoelectric body, and also relates to a manufacturing method of the fluid pump. This fluid pump is referred to as a piezoelectric pump in this specification, hereinafter.
This type piezoelectric pump is useful when a minute quantity of fluid is accurately conveyed or a minute quantity of fluid is controlled so that it can be accurately discharged from the pump. However, in an ink jet type printer which is a typical example to which the piezoelectric pump is applied, a bubble jet type printer using heated vapor is evaluated to be more economical and compact than a printer in which the piezoelectric type pump is used. However, as compared with the bubble jet type printer in which a heating means is used for generating vapor pressure, the piezoelectric type pump is superior from the viewpoint of energy efficiency. Accordingly, it has been desired to make the piezoelectric pump compact
2. Description of the Related Art
An amount of deformation of the piezoelectric element is very small. Therefore, when the piezoelectric element is put into practical use, an amplifying mechanism to amplify the deformation is used together with the piezoelectric element in many cases. A bimorph is a typical amplifying mechanism to amplify a displacement. In an amplifying section of the bimorph or other mechanisms, the rigidity is insufficient, so that the frequency characteristic is deteriorated and the speed of response is lowered. This problem is a factor to deteriorate the competitive power of the fluid pump in which the piezoelectric element is used. In order to increase the rigidity of the amplifying mechanism to amplify a displacement, concerning the oscillation mode, it is preferable to adopt an oscillation mode in the longitudinal oscillating direction.
With respect to the deformation of a piezoelectric body, the polarizing direction is referred to as d.sub.33, and a direction perpendicular to the polarizing direction is referred to as d.sub.31. When voltage E is applied in the polarizing direction and the thickness in the polarizing direction is represented by t, the electric field is represented by E/t. In the case where no load is given, the deformation in the thickness direction (the polarizing direction) caused by the electric field E/t is expressed as follows. EQU (E/t).times.d.sub.33 .times.t=Ed.sub.33
Therefore, the deformation is irrespective of the thickness t. On the other hand, in the direction of d.sub.31, when the length in the direction perpendicular to the thickness direction is represented by L, the deformation in the direction perpendicular to the thickness direction is expressed as follows. EQU (E/t).times.d.sub.31 .times.L
As shown in the above expression, it is possible to provide a large amount of deformation when L/t is appropriately selected.
An amount of the deformation d.sub.33 in the polarizing direction is substantially twice as large as that of the normal deformation d.sub.31. In the case of a piezoelement, the amount of the deformation d.sub.33 in the polarizing direction is approximately 600.times.10.sup.-12 m/V. However, due to the restriction of a semiconductor provided in the drive circuit, only several tens voltage is allowed for the deformation in the polarizing direction. Accordingly, an amount of the obtained displacement is only 0.0X .mu.m, so that the design of the device is difficult. In order to solve the above problem, an amplifying mechanism including a bimorph is employed, or alternatively a value of L/t is increased in the direction of d.sub.31 in the process of designing. However, even if the above countermeasure is taken, it is impossible to avoid a problem of slow speed of response.
When a high voltage is provided by a method in which a transformer is used, the circumstances are different, and it is possible to obtain a predetermined amount of displacement without using an amplifying mechanism.
Concerning the mechanism by which fluid is given pressure without using an amplifying mechanism, the simplest method is to dip a piezoelectric body in fluid as it is so that a change in the volume of the piezoelectric body is directly transmitted to fluid. This technique is disclosed in the U.S. Pat. No. 4,752,788. However, the piezoelectric body is deformed in such a manner that the piezoelectric body is contracted in the direction of d.sub.31 when the piezoelectric body is elongated in the direction of d.sub.33. Accordingly, an overall change in the volume is small. Japanese Unexamined Patent Publication (Kokai) No. 4-341835 is based on the concept that it is difficult to obtain a practical amount of contraction unless a large amount of deformation is provided by impressing a low voltage even in the case of a laminated piezoelectric body.
Japanese Unexamined Patent Publication (Kokai) No. 5-169657 discloses an arrangement in which pressure of fluid is raised and lowered when a piezoelectric body pushes a pressure chamber from the outside. However, in order to obtain a larger amount of displacement, a design is put into practical use, in which a value of L/t is increased using a displacement in the direction of d.sub.31.